The present invention relates to a picture-encoding apparatus and a picture-encoding method, which are used for encoding data in accordance with a format conforming to JPEG (Joint Photographic Experts Group)-2000 standard. The picture-encoding apparatus and the picture-encoding method are mainly applied to fields including a static-picture and moving-picture digital cameras, a cam-coder, a monitoring codec, a codec for broadcasting video equipment, a codec of a non-linear editor, a codec embedded in a PDA (Personal Digital Assistance) or embedded in a hand phone, an authoring tool running on a PC (Personal Computer), picture-editing software, a game machine and a compressor of a texture used in a three-dimensional CG (Computer Graphic) or its software module.
As a representative conventional picture compression technology, there has been made available a JPEG system standardized by an ISO (International Organization for Standardization). The JPEG system is known as a method for producing a good encoded/decoded picture by applying a DCT (Discrete Cosine Transform) technique for a case in which a relatively large number of bits are allocated. As the number of encoded bits is reduced to a certain degree, however, block distortion inherent in the DCT technique becomes prominent, making deterioration subjectively striking.
Separately from the JPEG system, there has been conducted many studies of encoding systems wherein a picture is divided by using the so-called filter bank into a plurality of bands and an encoding process is carried out for each of the bands. The filter bank is a combination of high-pass and low-pass filters. One of the encoding systems being studied is a wavelet encoding technique which is much regarded as a new technology replacing the DCT technique due to the fact that the wavelet encoding technique does not have a shortcoming of striking block distortion due to high compression as encountered as a problem in the DCT technique.
In accordance with the JPEG-2000 standard with its completion scheduled for December 2000, this wavelet transformation method adopts a means combining highly efficient entropy encoding technique and an arithmetic encoding technique. This means offers a much improved encoding efficiency in comparison with the JPEG technique. However, these international standards prescribe specifications on the decoder side only, hence allowing free designs on the encoder side. On the other side of the coin, there are neither specifications prescribing a means for reducing the amount of processing carried out by the encoder, which generally bears a heavy load, nor specifications prescribing effective means for rate control described in the specification of the present invention. For this reason, the establishment of such know-how is more important than anything. In addition, with the JPEG technique, rate control for realizing a target compression rate is difficult to execute. In many cases, it is necessary to carry out an encoding process a number of times till a target value is achieved. Since execution of an encoding process a number of times entails a long processing time, it becomes desirable to achieve a target encoded-data quantity by carrying out an encoding process only once.